1. Field of the Invention
The present invention relates to a device load variation protection circuit and, more particularly, to a circuit for protecting a device from a variation in a load for the device.
2. Description of the Prior Art
For example, a portable radio unit uses a power amplifier to amplify the transmission output. In a device such as a power amplifier, the characteristics of the device itself very greatly due to variations in a load impedance connected to the output terminal of the device. This may result in an increase in current consumption, degradation in power output efficiency, or damage to the device.
Under these circumstances, as a conventional countermeasure a load matching device such as an isolator is inserted to stabilize the output load of a device.
In another prior art disclosed in Japanese Unexamined Patent Publication No. 6-252785, a device is protected from a load variation as follows. In this prior art, the state of an antenna (the presence/absence of an antenna output) is detected, and the power supply of the device itself is switched for protection from a load variation.
However, in the above arrangement having a load matching device such as an isolator, the isolator itself has an attenuation amount, and the output power of the device itself must be made larger to obtain a desired radiation power as an antenna output. However, since it is not easy to increase the output power of the device, the increase in radiation power results in an increase in current consumption.
In the prior art disclosed in Japanese Unexamined Patent Publication No. 6-252785, the power supply of the device is instantaneously switched, and this is not preferable from the viewpoint of reliability or service life of the device.